The process of intrauterine growth retardation caused by maternal dietary restriction during gestation will be studied in ovine fetuses. During the last trimester of gestation, pregnant ewes will be fed diets which supply 100% (adequately nourished) or 58% (malnourished) of the daily recommended feed intake for pregnant ewes. Blood samples will be obtained during the last quarter of gestation from the fetuses by means of chronic indwelling cannulae. Blood plasma levels and secretory patterns of growth hormone, thyroid stimulating hormone and prolactin will be determined by radioimmunoassay for the fetus and dam. To determine if the nutritional status of the dam influences the fetal pituitary responsiveness to stimuli known to cause release of growth hormone, prolactin and thyroid stimulating hormone in adult sheep, the influences of arginine infusions and thyrotropin-releasing hormone injections will be studied in the fetuses of adequately and malnourished dams.